Saving track metadata format information for tracks demoted from cache for use when the demoted track is later staged into cache

ABSTRACT

Provided are a computer program product, system, and method for saving track metadata format information for tracks demoted from cache for use when the demoted track is later staged into cache. A track is demoted from the cache and indicated in a demoted track list. The track format information for the demoted track is saved, wherein the track format information indicates a layout of data in the track. The saved track format information for the demoted track is used when the demoted track is staged back into the cache.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a computer program product, system, and method for saving track metadata format information for tracks demoted from cache for use when the demoted track is later staged into cache.

2. Description of the Related Art

In a storage environment, a host system may communicate a read/write request to a connected storage system over network channel through a network adaptor. If the data is in a cache of the storage system, i.e., a read hit, then the data may be returned quickly to the host system. This reduces the delay in returning requested data to a host I/O request. However, if the requested data is not in the cache of the storage system, then there may be significant latency realized while the storage system needs to retrieve the requested data from storage to return. Further, the thread or task executing the host read request may have to be context switched and deactivated in order to allow the host system to process further I/O requests. When the data is returned to the read request, then the task must be reactivated and data for the task must be returned to registers and processor cache to allow processing of the returned data for the read request.

There is a need in the art for improved techniques for processing host read/write requests to the cache.

SUMMARY

Provided are a computer program product, system, and method for saving track metadata format information for tracks demoted from cache for use when the demoted track is later staged into cache. A track is demoted from the cache and indicated in a demoted track list. The track format information for the demoted track is saved, wherein the track format information indicates a layout of data in the track. The saved track format information for the demoted track is used when the demoted track is staged back into the cache.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an embodiment of a storage environment.

FIG. 2 illustrates an embodiment of a track format table entry.

FIG. 3 illustrates an embodiment of a cache control block.

FIG. 4 illustrates an embodiment of a cache Least Recently Used (LRU) list of tracks in the cache.

FIG. 5 illustrates an embodiment of a demoted cache Least Recently Used (LRU) list of tracks demoted from the cache.

FIG. 6 illustrates an embodiment of a demoted cache control block.

FIG. 7 illustrates an embodiment of a demoted cache control block directory entry.

FIG. 8 illustrates an embodiment of operations to process a read/write request received on a first channel, such as a bus interface.

FIG. 9 illustrates receive an embodiment of operations to process a read/write request received on a second channel, such as a network.

FIGS. 10a, 10b, and 10c illustrate an embodiment of operations to stage a track into the cache.

FIG. 11 illustrates an embodiment of operations to close track metadata and determine a track format code for the track in cache of the closed track metadata.

FIG. 12 illustrates an embodiment of a computer architecture used with described embodiments.

DETAILED DESCRIPTION

In a storage environment, a host system may first communicate a read/write request to a connected storage system over a fast channel, such as a bus interface, such as the Peripheral Component Interconnect Express (PCIe) interface. For a read/write request over the fast channel which is supposed to complete within a threshold time, the host system holds the application thread for the read/write request in a spin loop waiting for the request to complete. This saves processor time associated with a context swap, which deactivates the thread and reactivates the thread in response to an interrupt when a response to the read/write request is received. If the data for the read/write request sent on the fast channel is not in cache, then the storage system may fail the read/write request and the host system may communicate the same read/write request over a storage area network via a host adaptor, which is slower than processing the I/O request over the bus, e.g., PCIe interface. Communicating the read/write request over the second channel requires the host system to perform a context switch from the task handling the read/write request to another task while waiting for the read/write request to complete. Context switching is costly because it requires the processor running the task to clear all registers and L1 and L2 caches for the new task, and then when completing the new task, reactivate the context switched task and return the state data to the registers and L1 and L2 caches for the task that was context switched while waiting for the read/write request to complete.

Certain read/write operations need to be completed within a threshold time, else they are failed. The storage system will have to access track metadata to process a request to a track. The track metadata provides information on the format of data and layout of records in the track that are needed in order to perform reads and writes to the track. However, the reading of the track metadata from the storage comprises a substantial portion of the latency in processing read/write request. Described embodiments provide improvements to cache technology that reduce cache operation latency by including a track format code in the cache control block for a track in the cache. This track format code may be used for fast access to the track format from a track format table without having to read the track metadata from storage. By eliminating the need to read the track metadata from a metadata track in storage to determine the track layout, described embodiments increase the likelihood that read/write requests on the first channel that need to be completed within a threshold time are completed by accessing the track layout information for a track from the track format table, associating track format codes with track format information for common track formats.

Described embodiments provide further improvements to cache computer technology by reducing cache latency for a track staged into cache that was previously demoted by saving track format metadata, such as the track format code, when a track is demoted from the cache. When the demoted track is later staged into cache, the track format metadata may be quickly determined by the track format information saved with the demoted track and included in a cache control block for the staged track without having to read the metadata for the staged track. Avoiding the need to read the track metadata for a staged track substantially reduces the latency and delays in staging a track and processing a read/write request to the staged track.

With described embodiments, a read/write request to a target track on a channel requiring that the request be completed within a threshold time is processed if the track format code for the target track is within the cache control block for the target track. Using the track format code to access the track format metadata from the track format table reduces the latency of access to the track format metadata to allow the read/write request to complete within the time threshold. This keeps the time the host thread is spinning on the read/write request task for the read/write request sent on the bus interface within an acceptable time threshold. However, if the cache control block for the target track does not have a valid track format code, then the read/write request on the first channel is failed because it is unlikely the read/write request can complete within the threshold time given that the track format metadata will have to be retrieved from the storage. Failing the read/write request on the first channel, causes the host to redrive the read/write request on the second channel. The processing of the read/write request on the second channel reads in the track metadata from the storage to allow for processing the read/write request and adding the track format code to the cache control block for the target track.

FIG. 1 illustrates an embodiment of a storage environment in which a host 100 directs read and write requests to a storage system 102 to access tracks in volumes configured in storage devices 104 in a disk enclosure 106. The host 100 includes a processor complex 108 of one or more processor devices and a memory 110 including an operating system 111 executed by the processor complex 108. The host operating system 111 generates read and write requests to tracks configured in the storage devices 104. The host 100 includes hardware to communicate read and write requests on two different channels. A first channel is a bus interface, such as a Peripheral Component Interconnect Express (PCIe), including a bus 112, a bus switch 114 to connect one or more devices on the bus 112, including the processor complex 108, a memory system 110, and a bus host adaptor 116 to extend the bus interface over an external bus interface cable 118 to the storage system 102. Additional bus interface technology to extend the bus interface may be utilized, including PCIe extender cables or components, such as a distributed PCIe switch, to allow PCIe over Ethernet, such as with the ExpEther technology. A second channel to connect the host 100 and storage system 102 uses a network host adaptor 120, connected to the bus 112, that connects to a separate network 122 over which the host 100 and storage system 102 additionally communicate. The first channel through the bus interface may comprise a faster access channel than the network 122 interface through the network host adaptor 120.

The storage system 102 includes a bus interface comprising a bus 124 a, 124 b, a bus switch 126 to connect to endpoint devices on the bus 124 a, 124 b, and a bus host adaptor 128 to connect to the external bus interface cable 118 to allow communication over the bus interface to the host 100 over the first channel. The storage system 102 includes an Input/Output bay 130 having the bus host adaptor 128, one or more device adaptors 132 to connect to the storage devices 104, and one or more network host adaptors 134 to connect to the network 122 and host systems.

The storage system 102 includes a processor complex 136 of one or more processor devices and a memory 138 having a cache 140 to cache tracks accessed by the connected hosts 100. The memory 138 includes a cache manager 142 and a storage manager 144. The storage manager 144 manages access requests from processes in the hosts 100 and storage system 102 for tracks in the storage 104. The devices 136, 138, 128, 132, and 134 connect over the bus interface implemented in the bus lanes 124 a, 124 b and bus switch 126.

The cache manager 142 maintains accessed tracks in the cache 140 for future read access to the tracks to allow the accessed tracks to be returned from the faster access cache 140 instead of having to retrieve from the storage 104. Further, tracks in the cache 140 may be updated by writes. A track may comprise any unit of data configured in the storage 104, such as a track, Logical Block Address (LBA), etc., which is part of a larger grouping of tracks, such as a volume, logical device, etc.

The cache manager 142 maintains cache management information 146 in the memory 138 to manage read (unmodified) and write (modified) tracks in the cache 140. The cache management information 146 may include a track format table 200 having track format codes for common track format descriptors providing details of a layout and format of data in a track; track index 148 providing an index of tracks in the cache 140 to cache control blocks in a control block directory 300; and a Least Recently Used (LRU) list 400 for tracks in the cache 140. The control block directory 300 includes the cache control blocks, where there is one cache control block for each track in the cache 140 providing metadata on the track in the cache 140. The track index 148 associates tracks with the cache control blocks providing information on the tracks in the cache 140. Upon determining that the cache LRU list 400 is full or has reached a threshold level, tracks are demoted from the LRU list 400 to make room for more tracks to stage into the cache 140 from the storage 104.

In certain embodiments, there may be multiple hosts 100 that connect to the storage system 102 over the first and second channels to access tracks in the storage devices 104. In such case, the storage system 102 would have at least one bus host adaptor 128 to connect to the bus interface 118 of each connected host 100 and one or more network host adaptors 134 to connect to the network host adaptors 120 on the hosts 100.

In one embodiment, the bus interfaces 112, 114, 116, 118, 124 a, 124 b, 126, and 128 may comprise a Peripheral Component Interconnect Express (PCIe) bus interface technology. In alternative embodiments, the bus interfaces 112, 114, 116, 118, 124 a, 124 b, 126, and 128 may utilize suitable bus interface technology other than PCIe. The bus host adaptors 116 and 128 may comprise PCIe host adaptors that provide the interface to connect to the PCIe cable 118. The network 122 may comprise a Storage Area Network (SAN), a Local Area Network (LAN), a Wide Area Network (WAN), the Internet, an Intranet, etc., and the network host adaptors 120, 134 provide the network 122 connections between the hosts 100 and storage system 102.

The storage system 102 may comprise a storage system, such as the International Business Machines Corporation (IBM®) D58000® and DS8880 storage systems, or storage controllers and storage systems from other vendors. (IBM and DS8000 are trademarks of International Business Machines Corporation throughout the world). The host operating system 111 may comprise an operating system such as Z Systems Operating System (Z/OS®) from International Business Machines Corporation (“IBM”) or other operating systems known in the art. (Z/OS is a registered trademark of IBM throughout the world).

The storage devices 104 in the disk enclosure 106 may comprise different types or classes of storage devices, such as magnetic hard disk drives, solid state storage device (SSD) comprised of solid state electronics, EEPROM (Electrically Erasable Programmable Read-Only Memory), flash memory, flash disk, Random Access Memory (RAM) drive, storage-class memory (SCM), etc., Phase Change Memory (PCM), resistive random access memory (RRAM), spin transfer torque memory (STT-RAM), conductive bridging RAM (CBRAM), magnetic hard disk drive, optical disk, tape, etc. Volumes in a storage space may further be configured from an array of devices, such as Just a Bunch of Disks (JBOD), Direct Access Storage Device (DASD), Redundant Array of Independent Disks (RAID) array, virtualization device, etc. Further, the storage devices 104 in the disk enclosure 106 may comprise heterogeneous storage devices from different vendors and different types of storage devices, such as a first type of storage devices, e.g., hard disk drives, that have a slower data transfer rate than a second type of storage devices, e.g., SSDs.

FIG. 2 illustrates an embodiment of a track format table entry 200 _(i) in the track format table 200, which includes a track format code 202 and the track format metadata 204. In certain embodiments Count Key Data (CKD) track embodiments, the track format metadata 204 may comprise a track format descriptor (TFD) indicating a number of records in the track, a block size, a number of blocks in the track, a data length of each of the records, and a control interval size indicating an amount of data that is read or written atomically as a unit, number of blocks in a control interval, and whether a control interval spans two tracks, and other information. The track format code 202 may comprise an index value of the index entry 200 _(i), in the track format table 200. For instance, if there are 32 track format table entries 200 _(i), then the track format code 202 may comprise 5 bits to reference the different possible number of 32 entries 200 _(i).

FIG. 3 illustrates an embodiment of a cache control block 300 _(i) for one of the tracks in the cache 140, including, but not limited to, a cache control block identifier 302, such as an index value of the cache control block 300 _(i); a track ID 304 of the track in the storage 104; the cache LRU list 306 in which the cache control block 300 _(i) is indicated; an LRU list entry 308 at which the track is indicated; a cache timestamp 310 indicating a time the track was added to the cache 140 and indicated on the LRU list 306; additional track metadata 312 typically maintained for tracks stored in the cache 140, such as a dirty flag indicating whether the track was modified; a track format code 314 comprising one of the track format codes 202 of the track format metadata 204 describing the layout of data in the track 304 represented by the cache control block 300 _(i); a track format code valid flag 316 indicating whether the track format code 314 is valid or invalid; and an invalid reason 318 indicating a reason for the track format code valid flag 316 code being invalid, as indicated in the track format code valid flag 316.

FIG. 4 illustrates an embodiment of an LRU list 400 _(i), such as having a most recently used (MRU) end 402 identifying a track most recently added to the cache 140 or most recently accessed in the cache 140 and a least recently used (LRU) end 404 from which the track identified at the LRU end 404 is selected to demote from the cache 140. The MRU end 402 and LRU end 404 point to track identifiers, such as a track identifier address or a cache control block index for the track, of the tracks that have been most recently added and in the cache 140 the longest, respectively, for tracks indicated in that list 400.

FIG. 5 illustrates an embodiment of the demoted cache LRU list 500, having a most recently used (MRU) end 502 identifying a demoted track most recently added to the demoted cache LRU list 500 and a least recently used (LRU) end 504 from which the demoted track identified at the LRU end 504 is selected to demote from the demoted cache LRU list 500.

FIG. 6 illustrates an embodiment of a demoted cache control block 600 _(i) having a track identifier (ID) 602 of a demoted track; a track format code 604 if available of the track format metadata 204 in the track format table 200 of the demoted track; a pointer to a previous LRU entry 606 of a previous demoted track in the demoted cache LRU list 500; a pointer to a next LRU entry 608 of a next demoted track in the demoted cache LRU list 500; and a pointer 610 to next demoted cache control block in the entry in the demoted cache control block directory 700 for the track ID 602.

In additional embodiments, the track format code 604 may comprise track format information other than a code 604 in a table 200, such as other information that may be used to identify or represent the track format metadata and layout of data in the tracks and comprises substantially less bits of information than the represented track format metadata and layout information.

FIG. 7 illustrates an embodiment of an entry 700 _(i) in the demoted cache control block 700 that includes pointers 702 ₁, 702 ₂ . . . 702 _(n) to demoted cache control blocks 600 _(i). Each demoted cache control block 600 _(i) maps to one entry in the demoted cache control block directory 700 based on the track ID 602. In one embodiment, a hash function of the track identifier would produce the entry in the demoted cache control block directory 700 in which the demoted cache control block 600 _(i) is indicated.

FIG. 8 illustrates an embodiment of operations performed by the cache manager 142 and storage manager 144 to process a read/write request to a target track received on a first fast channel, such as the PCIe bus interface via bus host adaptor 128. Upon receiving (at block 800) the read/write request at the bus host adaptor 128, if (at block 802) the target track is not in the cache 140, then the storage manager 144 returns (at block 804) fail to the read/write request on the first channel or bus host adaptor 128 to the host 100, which causes the host 100 to retry the read/write request on the second channel or network host adaptor 120, 134. Failure is returned because if the target track is not in the cache 140, then the target track and track metadata needs to be staged into cache 140, which would likely exceed the time threshold for read/writes on the first channel, where the host processor is spinning on the thread of the read/write request. If (at block 802) the target track is in the cache 140 is a write and if (at block 808) the write modifies the track format, then the cache manager 142 sets (at block 810) the track format code valid flag 316 to invalid and indicates (at block 812) the invalid reason 318 that the track in the cache 140 was invalidated as track format change. The storage manager 144 then returns (at block 804) fail to the host 100 because the track metadata needs to be read from the storage 104 to update with the modified track format.

If (at block 806) the read/write request is a read or if (at block 808) the request is a write that does not modify the track format, then the cache manager 142 determines (at block 814) if the track format code valid flag 316 is set to valid. If so, then the cache manager 142 determines (at block 816) the track format metadata 204 in the track format table 200 corresponding to the track format code 314 in the cache control block 300 _(i). The cache manager 142 uses (at block 818) the track format layout indicated in the determined track format metadata 204 to process the read or write request to the target track in the cache 140. If the request is a write, a dirty flag 312 in the cache control block 300 _(i) may be set to indicate the track is modified. If (at block 814) the track format code valid flag 316 is invalid, meaning there is no fast access to track format information available through the track format code 314, then the storage manager 144 returns (at block 804) fail on the bus interface to the bus host adaptor 128 because the track format table 200 cannot be used, and the track metadata needs to be read from the storage 104, which would introduce too much latency for the fast read/write on the first channel.

With the embodiment of operations of FIG. 8, during a fast write over the bus interface or first channel, if the track format metadata may be accessed without latency through the track format table 200, then the read/write request is allowed to proceed when the transaction can be processed very quickly because the track metadata can be obtained directly from the track format table 200 through the track format code 314, without having to read the track metadata from storage 104. However, if the cache control block 300 _(i) does not have a valid track format code 314 to allow low latency access of track format metadata, then the read/write request is failed because the transaction will not likely complete within a fast time threshold. This determination is important to avoid host delays in processing other tasks while the host processor is spinning on the thread handling the read/write request while waiting for the read/write request to complete. If the track metadata can be accessed from the track format table 200 than there is a high likelihood the read/write can complete on the bus interface channel within the time required to avoid the host processor holding the thread for too long, which causes other I/O requests to be queued and delayed. If the track metadata cannot be accessed from the track format table 200 and needs to be read from the storage 104, then it is unlikely the read/write request will complete within the time threshold for the host processor to spin on the thread for the read/write request, and failure is returned. Returning failure when the track metadata cannot be obtained from the track format table 200 causes the host thread waiting on the read/write request task to be deactivated and the host processor may context switch to processing other tasks, and then the read/write request is retried on the second network channel during the context switch.

FIG. 9 illustrates an embodiment of operations performed by the cache manager 142 and storage manager 144 to process a read/write request to a target track received on a second channel, such as the network 122 on network host adaptor 134. Upon receiving (at block 900) the read/write request, if (at block 902) the target track is not in the cache 140, then the cache manager 142 proceeds (at block 904) to block 1000 in FIG. 10a to stage the track into the cache 140. If (at block 908) the read/write request is a write and if (at block 910) the write modifies the track format, then the cache manager 142 updates (at block 912) the track metadata to indicate the modified track format and sets (at block 914) the track format code valid flag 316 to invalid. The track metadata 312 is further updated (at block 916) to indicate the track is modified or dirty. If (at block 908) the request is a read or from block 916, the cache manager 142 uses (at block 918) the track format layout indicated in the track format metadata to process the read or write request to the target track in the cache 140.

If (at block 902) the target track is in the cache 140 and if (at block 930) the track format code valid flag 316 is set to valid, then the cache manager 142 determines (at block 932) the track format metadata 204 in the track format table 200 corresponding to the track format code 314 in the cache control block 300 _(i) for the target track. From block 932, control proceeds to block 908 to process the read/write request. If (at block 930) the track format code valid flag 316 is set to invalid, then the cache manager 142 reads (at block 934) the track metadata for the target track from the storage 104 to determine the track format, e.g., size of blocks, control interval, layout of records on the track, etc. From block 934, control proceeds to block 908 to process the read/write request.

With the embodiment of FIG. 9, when the read/write request is received on the second slower channel, such as over the network 122, where the host operating system 111 would have performed a context switch for the thread handling the read/write request, the cache manager 142 may read the track metadata from the storage 104 to determine the track layout to process the request. During this time, the host processing of further host requests is not delayed because the host thread handling the read/write request is context switched and not active, until the read/write request returns complete.

FIGS. 10a, 10b, and 10c illustrate an embodiment of operations performed by the cache manager 142 to stage a track into the cache 140, which may be invoked at block 904 in FIG. 9 when the target track of a read/write request is not in the cache 140. Upon initiating (at block 1000) the operation to stage a track into the cache 140, if (at block 1002) the cache LRU list 400 is full, then the track at the LRU end 404 of the cache LRU list 400 is selected (at block 1004) to demote. If (at block 1006) the demoted cache LRU list 500 is full, then the cache manager 142 selects (at block 1008) a demoted track indicated at the LRU end 504 of the demoted cache LRU list 500 to demote. The selected demoted track is removed (at block 1010) from the LRU end 504. The cache manager 142 adjusts (at block 1012) a demoted cache control block 600 _(j) whose pointer to next LRU entry 608 points to the selected demoted track in the demoted cache LRU list 500 to indicate that the pointer 608 is null, because now that entry is at the LRU end 504 when the selected demoted track is removed from the demoted cache LRU list 500.

The cache manager 142 determines (at block 1014) an entry 700 _(i) in the demoted cache control block directory 700 having the demoted cache control block 600 _(S) of the selected demoted track. In one embodiment, the entry 700 _(i) may be determined by applying a hash function to the selected demoted track identifier. The hash function may map any of the track identifiers in the storage 104 to one of the entries 700 _(i) in the demoted cache control block directory 700. The cache manager 142 then needs to adjust the pointer 610 that points to the demoted cache control block 600 _(S) of the selected demoted track. For this, the cache manager 142 adjusts (at block 1016) a demoted cache control block 600 _(j) in the determined entry 700 _(i) that points to the demoted cache control block 600 _(S) of the selected demoted track to point to the demoted cache control block 600 _(k) pointed to by the pointer 610 of the demoted cache control block 600 _(S) of the selected demoted track. The demoted cache control block 600 _(S) for the selected demoted track is deleted (at block 1018) and indication of the deleted demoted cache control block 600 _(S) is removed (at block 1020) from the entry 700 _(i) in the demoted cache control block directory 700.

From block 1020 or if (at block 1006) the demoted cache LRU list 500 is not full, control proceeds (at block 1022) to block 1030 in FIG. 10b to add the demoted track from the cache 140 to the demoted cache LRU list 500. Upon initiating (at block 1030) the operation to add the demoted track to the demoted cache LRU list 500, the cache manager 142 indicates (at block 1032) the demoted track at the MRU end 502 of the demoted cache LRU list 500. The cache manager 142 determines (at block 1034) the cache control block 300 _(DT) for the demoted track from the cache 140 and the track format code 314 for the demoted track if one is included. The cache manager 142 generates (at block 1036) a demoted track cache control block 600 _(DT) for the track being demoted indicating a track identifier 602 of the demoted track and the determined track format code 314 for the demoted track to save the track format code in field 604 of the demoted track cache control block 600 _(DT). If there was no valid track format code 314 in the cache control block 300 _(DT), then a null value may be indicated in the field 604 indicating there is no valid track format code for the demoted track.

The cache manager 142 determines (at block 1038) the entry 700 _(i) in the demoted cache control block directory 700 that will be used to indicate the demoted cache control block 600 _(DT) of the demoted track. The next pointer 610 of the last demoted cache control block 600 _(i) indicated in the determined entry 700 _(i) is adjusted (at block 1040) to point to the demoted cache control block 600 _(DT) for the track being demoted. The demoted track control block 600 _(DT) for the demoted track is indicated (at block 1042) in the determined entry 700 _(i) in the demoted cache control block directory 700, e.g., at the end of the entry 700 _(i). After adding the demoted track to the demoted LRU list 500 and the demoted cache control block 600 _(DT) to the demoted cache control block directory 700, control proceeds (at block 1044) to block 1060 in FIG. 10c to stage the target track to the cache 140. Control also proceeds to block 1060 in FIG. 10c to stage the track if (at block 1002 in FIG. 10a ) the cache LRU list 400 is not full, so that a track does not need to be demoted from the cache 140 and added to the demoted cache LRU list 500.

Upon initiating (at block 1060) the operation to stage the track to the cache 140, the cache manager 142 stages (at block 1062) the target track into the cache 140 from the storage 104. The staging of the actual track data from the storage 104 may have been initiated earlier before or during the operations of FIGS. 10a, 10b to manage the demote track LRU list 500 and demote cache control block directory 700. The target track staged into the cache 140 is indicated (at block 1064) at the MRU end 402 of the cache LRU list 400. A cache control block 300 _(ST) is generated (at block 1066) for the staged track. Control then proceeds to block 1068 read to determine if there is a demoted cache control block 600 _(ST) for the staged track that has a track format code 604 (or other track format information) that can be included in the cache control block 300 _(ST) created for the staged track. A determination is made (at block 1068) of the entry 700 _(i) in the demoted cache control block directory 700 that could have a demoted cache control block 600 _(ST) for the staged track, which entry 700 _(i) may be determined by applying a hash function to the track identifier of the staged track. The first demoted cache control block 600 _(SEL) in the determined entry 700 _(i) is selected (at block 1070). If (at block 1072) the track identifier 602 of the selected demoted cache control block 600 _(SEL) matches the track identifier of the staged track, then the track format code 604 in the selected demoted cache control block 600 _(SEL) is included (at block 1074) in the cache control block 300 _(ST) for the staged track. The cache manager 142 may then perform (at block 1076) the operations at blocks 1010, 1016, 1018, and 1020 in FIG. 10a to remove demoted track information for the staged track, including removing the demoted cache control block 600 _(ST) for the staged track, removing the staged track from the demoted cache LRU list 500, removing the indication of the demoted cache control block 600 _(ST) from the demoted cache control block directory 700, and adjusting pointers 606, 608, 610 in other demoted cache control blocks 600 _(i) that pointed to the demoted track or demoted cache control block 600 _(ST) for the staged track because the staged track is no longer demoted but active in cache 140.

If (at block 1072) the selected demoted cache control block 600 _(SEL) is not for the staged track and if (at block 1078) there is a next demoted cache control block 600 _(i) in the entry 700 _(i), which may be indicated in the pointer 610, then that next demoted cache control block is selected (at block 1080) and control returns to block 1070 to determine whether this next demoted cache control block 600 _(i) is for the staged track. If (at block 1078) there are no further next demoted cache control blocks in the determined entry 700 _(i) to consider, then the track format code 202 from a demoted track information cannot be used and the cache manager 142 reads (at block 1082) the metadata for the track from the storage 104 to determine the track format. From block 1076 after using the track format code 604 from the demoted cache control block for the staged track or after reading (at block 1082) the metadata for the staged track, control returns (at block 1084) to block 904 in FIG. 9 with staging complete to perform the read/write operation with respect to the staged track.

With the embodiments of FIGS. 10a, 10b, and 10c , the track format code for a track demoted from cache can be saved and later used when the demoted track is staged back into cache. This allows the track metadata format to be quickly determined for the demoted track staged back into cache without having to read the metadata for the track from storage. The computer performance for cache operations, particularly staging, are substantially improved and latency reduced by determining the track metadata format and layout of a track being staged into cache without having to read the metadata for the track.

FIG. 11 illustrates an embodiment of operations performed by the cache manager 142 when closing the track metadata for a track in the cache 140, which involves destaging the track metadata to the storage 104 if changed. Upon closing (at block 1100) the track metadata for a track in the cache 140, the cache manager 142 processes (at block 1102) the track metadata to determine a track format or a layout of data in the track. If (at block 1104) the track format table 200 does not have a track format 204 matching the determined track format from the track metadata, which may happen if the determined track format is irregular, then the track format code valid flag 316 is set (at block 1106) to invalid and the invalid reason 318 is set to indicate that the track format is not supported. In such situation, read/write requests to the track having an irregular format are only processed when received through the second channel via network host adaptor 134.

If (at block 1104) the track format table has a track format 204 matching the determined track format from the track metadata, then the cache manager 142 determines the track format code 202 for the determined track format 204 in the track format table 200 and includes the track format code 202 in the field 314 in the cache control block 300 _(i). The track format code valid flag 316 is set (at block 1116) to valid. From block 1108 or 1116, control proceeds to block 1118 to destage the track metadata from the memory 138 if modified or discard if not modified.

With the operations of FIG. 11, the track format information may be indicated in the cache control block 300 _(i) with a track format code 202 having a limited number of bits to index track format metadata 204 describing track layout in a track format table 200, where the track metadata itself would not fit into the cache control block 300 _(i). For future read/write accesses, if a valid track format code 314 is provided, then the cache manager 142 may use that code 314 to obtain with low latency the track format metadata 204 from the track format table 200 without having to read the track metadata from the storage 104 and process to determine the track format.

The present invention may be implemented as a system, a method, and/or a computer program product. The computer program product may include a computer readable storage medium (or media) having computer readable program instructions thereon for causing a processor to carry out aspects of the present invention.

The computer readable storage medium can be a tangible device that can retain and store instructions for use by an instruction execution device. The computer readable storage medium may be, for example, but is not limited to, an electronic storage device, a magnetic storage device, an optical storage device, an electromagnetic storage device, a semiconductor storage device, or any suitable combination of the foregoing. A non-exhaustive list of more specific examples of the computer readable storage medium includes the following: a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), a static random access memory (SRAM), a portable compact disc read-only memory (CD-ROM), a digital versatile disk (DVD), a memory stick, a floppy disk, a mechanically encoded device such as punch-cards or raised structures in a groove having instructions recorded thereon, and any suitable combination of the foregoing. A computer readable storage medium, as used herein, is not to be construed as being transitory signals per se, such as radio waves or other freely propagating electromagnetic waves, electromagnetic waves propagating through a waveguide or other transmission media (e.g., light pulses passing through a fiber-optic cable), or electrical signals transmitted through a wire.

Computer readable program instructions described herein can be downloaded to respective computing/processing devices from a computer readable storage medium or to an external computer or external storage device via a network, for example, the Internet, a local area network, a wide area network and/or a wireless network. The network may comprise copper transmission cables, optical transmission fibers, wireless transmission, routers, firewalls, switches, gateway computers and/or edge servers. A network adapter card or network interface in each computing/processing device receives computer readable program instructions from the network and forwards the computer readable program instructions for storage in a computer readable storage medium within the respective computing/processing device.

Computer readable program instructions for carrying out operations of the present invention may be assembler instructions, instruction-set-architecture (ISA) instructions, machine instructions, machine dependent instructions, microcode, firmware instructions, state-setting data, or either source code or object code written in any combination of one or more programming languages, including an object oriented programming language such as Java, Smalltalk, C++ or the like, and conventional procedural programming languages, such as the “C” programming language or similar programming languages. The computer readable program instructions may execute entirely on the user's computer, partly on the user's computer, as a stand-alone software package, partly on the user's computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user's computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider). In some embodiments, electronic circuitry including, for example, programmable logic circuitry, field-programmable gate arrays (FPGA), or programmable logic arrays (PLA) may execute the computer readable program instructions by utilizing state information of the computer readable program instructions to personalize the electronic circuitry, in order to perform aspects of the present invention.

Aspects of the present invention are described herein with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer readable program instructions.

These computer readable program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. These computer readable program instructions may also be stored in a computer readable storage medium that can direct a computer, a programmable data processing apparatus, and/or other devices to function in a particular manner, such that the computer readable storage medium having instructions stored therein comprises an article of manufacture including instructions which implement aspects of the function/act specified in the flowchart and/or block diagram block or blocks.

The computer readable program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other device to cause a series of operational steps to be performed on the computer, other programmable apparatus or other device to produce a computer implemented process, such that the instructions which execute on the computer, other programmable apparatus, or other device implement the functions/acts specified in the flowchart and/or block diagram block or blocks.

The flowchart and block diagrams in the Figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer program products according to various embodiments of the present invention. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of instructions, which comprises one or more executable instructions for implementing the specified logical function(s). In some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts or carry out combinations of special purpose hardware and computer instructions.

The computational components of FIG. 1, including the host 100 and storage system 102 may be implemented in one or more computer systems, such as the computer system 1202 shown in FIG. 12. Computer system/server 1202 may be described in the general context of computer system executable instructions, such as program modules, being executed by a computer system. Generally, program modules may include routines, programs, objects, components, logic, data structures, and so on that perform particular tasks or implement particular abstract data types. Computer system/server 1202 may be practiced in distributed cloud computing environments where tasks are performed by remote processing devices that are linked through a communications network. In a distributed cloud computing environment, program modules may be located in both local and remote computer system storage media including memory storage devices.

As shown in FIG. 12, the computer system/server 1202 is shown in the form of a general-purpose computing device. The components of computer system/server 1202 may include, but are not limited to, one or more processors or processing units 1204, a system memory 1206, and a bus 1208 that couples various system components including system memory 1206 to processor 1204. Bus 1208 represents one or more of any of several types of bus structures, including a memory bus or memory controller, a peripheral bus, an accelerated graphics port, and a processor or local bus using any of a variety of bus architectures. By way of example, and not limitation, such architectures include Industry Standard Architecture (ISA) bus, Micro Channel Architecture (MCA) bus, Enhanced ISA (EISA) bus, Video Electronics Standards Association (VESA) local bus, and Peripheral Component Interconnects (PCI) bus.

Computer system/server 1202 typically includes a variety of computer system readable media. Such media may be any available media that is accessible by computer system/server 1202, and it includes both volatile and non-volatile media, removable and non-removable media.

System memory 1206 can include computer system readable media in the form of volatile memory, such as random access memory (RAM) 1210 and/or cache memory 1212. Computer system/server 1202 may further include other removable/non-removable, volatile/non-volatile computer system storage media. By way of example only, storage system 1213 can be provided for reading from and writing to a non-removable, non-volatile magnetic media (not shown and typically called a “hard drive”). Although not shown, a magnetic disk drive for reading from and writing to a removable, non-volatile magnetic disk (e.g., a “floppy disk”), and an optical disk drive for reading from or writing to a removable, non-volatile optical disk such as a CD-ROM, DVD-ROM or other optical media can be provided. In such instances, each can be connected to bus 1208 by one or more data media interfaces. As will be further depicted and described below, memory 1206 may include at least one program product having a set (e.g., at least one) of program modules that are configured to carry out the functions of embodiments of the invention.

Program/utility 1214, having a set (at least one) of program modules 1216, may be stored in memory 806 by way of example, and not limitation, as well as an operating system, one or more application programs, other program modules, and program data. Each of the operating system, one or more application programs, other program modules, and program data or some combination thereof, may include an implementation of a networking environment. The components of the computer 1202 may be implemented as program modules 1216 which generally carry out the functions and/or methodologies of embodiments of the invention as described herein. The systems of FIG. 1 may be implemented in one or more computer systems 1202, where if they are implemented in multiple computer systems 1202, then the computer systems may communicate over a network.

Computer system/server 1202 may also communicate with one or more external devices 1218 such as a keyboard, a pointing device, a display 1220, etc.; one or more devices that enable a user to interact with computer system/server 1202; and/or any devices (e.g., network card, modem, etc.) that enable computer system/server 1202 to communicate with one or more other computing devices. Such communication can occur via Input/Output (I/O) interfaces 1222. Still yet, computer system/server 1202 can communicate with one or more networks such as a local area network (LAN), a general wide area network (WAN), and/or a public network (e.g., the Internet) via network adapter 1224. As depicted, network adapter 1224 communicates with the other components of computer system/server 1202 via bus 1208. It should be understood that although not shown, other hardware and/or software components could be used in conjunction with computer system/server 1202. Examples, include, but are not limited to: microcode, device drivers, redundant processing units, external disk drive arrays, RAID systems, tape drives, and data archival storage systems, etc.

The terms “an embodiment”, “embodiment”, “embodiments”, “the embodiment”, “the embodiments”, “one or more embodiments”, “some embodiments”, and “one embodiment” mean “one or more (but not all) embodiments of the present invention(s)” unless expressly specified otherwise.

The terms “including”, “comprising”, “having” and variations thereof mean “including but not limited to”, unless expressly specified otherwise.

The enumerated listing of items does not imply that any or all of the items are mutually exclusive, unless expressly specified otherwise.

The terms “a”, “an” and “the” mean “one or more”, unless expressly specified otherwise.

Devices that are in communication with each other need not be in continuous communication with each other, unless expressly specified otherwise. In addition, devices that are in communication with each other may communicate directly or indirectly through one or more intermediaries.

A description of an embodiment with several components in communication with each other does not imply that all such components are required. On the contrary a variety of optional components are described to illustrate the wide variety of possible embodiments of the present invention.

When a single device or article is described herein, it will be readily apparent that more than one device/article (whether or not they cooperate) may be used in place of a single device/article. Similarly, where more than one device or article is described herein (whether or not they cooperate), it will be readily apparent that a single device/article may be used in place of the more than one device or article or a different number of devices/articles may be used instead of the shown number of devices or programs. The functionality and/or the features of a device may be alternatively embodied by one or more other devices which are not explicitly described as having such functionality/features. Thus, other embodiments of the present invention need not include the device itself.

The foregoing description of various embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto. The above specification, examples and data provide a complete description of the manufacture and use of the composition of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims herein after appended. 

What is claimed is:
 1. A computer program product for managing tracks in storage cached in a cache, the computer program product comprising a computer readable storage medium having computer readable program code embodied therein that is executable to perform operations, the operations comprising: demoting a track from the cache; indicating the demoted track in a demoted track list; saving track format information for the demoted track, wherein the track format information indicates a layout of data in the track; and using the saved track format information for the demoted track when the demoted track is staged back into the cache.
 2. The computer program product of claim 1, wherein the track format information comprises a track format code defined in a track format table associating track format codes with track format metadata.
 3. The computer program product of claim 1, wherein the track format information comprises a track format code defined in a track format table associating track format codes with track format metadata, wherein the using the saved track format information for the demoted track when the demoted track is staged back into the cache comprises: staging a track from the storage into the cache; generating a cache control block for the staged track; determining whether there is a saved track format information for the staged track that was saved when the staged track was previously demoted; and including the saved track format information for the staged track in the cache control block for the staged track in response to determining that there is the saved format track information for the staged track.
 4. The computer program product of claim 3, wherein the operations further perform in response to determining that there is no saved track format code for the staged track: processing track format metadata for the staged track; determining whether the track format table has track format metadata matching the track format metadata of the track staged to the cache; determining a track format code from the track format table for the track format metadata in the track format table matching the track format metadata of the staged track in response to the track format table having the matching track format metadata; and including the determined track format code in the cache control block for the staged track.
 5. The computer program product of claim 1, wherein the operations further comprise: determining whether the demoted track list is full; in response to determining that the demoted track list is full, performing: removing indication of a demoted track in the demoted track list; and deleting a saved track format information for the demoted track removed from the demoted track list, wherein the demoted track is added to the demoted track list after removing the indication of the demoted track from the demoted track list.
 6. The computer program product of claim 1, wherein the saving the track format information for the demoted track comprises generating a demoted track cache control block indicating the demoted track and indicating the track format information for the demoted track, and wherein using the saved track format information comprises: generating a cache control block for a track staged into the cache; determining whether there is a demoted cache control block for the track staged into the cache; and including the track format information indicated in the demoted cache control block for the track staged into the cache in the cache control block for the track staged into the cache.
 7. The computer program product of claim 6, wherein the operations further comprise: deleting the demoted cache control block and removing indication of the demoted track from the demoted track list for the track staged into cache in response to including the track format information in the demoted cache control block in the generated cache control block for the track staged into the cache.
 8. The computer program product of claim 1, wherein the saving the track format information for the demoted track comprises generating a demoted cache control block indicating a track identifier of the demoted track and indicating the track format information for the demoted track, and wherein the operations further comprise: determining an entry in a demoted cache control block directory, having a plurality of entries, based on a track identifier of the demoted track; and indicating the demoted cache control block in the determined entry in the demoted cache control block directory.
 9. The computer program product of claim 8, wherein the operations further comprise: performing a hash function on the track identifier of the demoted track to determine the entry in the demoted cache control block directory for the demoted cache control block for the demoted track.
 10. The computer program product of claim 8, wherein each of a plurality of entries in the demoted cache control block directory indicate a plurality of demoted cache control blocks whose track identifiers map to the entry in the demoted cache control block directory, wherein the using the saved track format information for the demoted track when the demoted track is staged back into the cache comprises: staging a track from the storage into the cache; generating a cache control block for the staged track; determine an entry of the entries in the demoted cache control block directory based on a track identifier of the staged track; processing indications of demoted cache control blocks in the determined entry for the staged track to determine whether one of the demoted cache control blocks indicated in the determined entry has a track identifier matching the track identifier of the staged track; and including the track format information in the demoted cache control block having the track identifier matching the track identifier of the staged track in the cache control block for the staged track in response to determining the demoted cache control block in the determined entry for the staged track.
 11. The computer program product of claim 10, wherein the operations further comprise in response to including the track format information in the demoted cache control block in the cache control block for the staged track: removing indication of the demoted cache control block for the staged track from the determined entry in the demoted cache control block directory; deleting the demoted cache control block for the staged track; and removing indication of the track in the deleted demoted cache control block from the demoted track list.
 12. A system for managing read and write requests from a host to tracks in storage, comprising: a processor; a cache to cache tracks from the storage; a computer readable storage medium having computer readable program code embodied therein that is executable to perform operations, the operations comprising: demoting a track from the cache; indicating the demoted track in a demoted track list; saving track format information for the demoted track, wherein the track format information indicates a layout of data in the track; and using the saved track format information for the demoted track when the demoted track is staged back into the cache.
 13. The system of claim 12, wherein the track format information comprises a track format code defined in a track format table associating track format codes with track format metadata.
 14. The system of claim 12, wherein the track format information comprises a track format code defined in a track format table associating track format codes with track format metadata, wherein the using the saved track format information for the demoted track when the demoted track is staged back into the cache comprises: staging a track from the storage into the cache; generating a cache control block for the staged track; determining whether there is a saved track format information for the staged track that was saved when the staged track was previously demoted; and including the saved track format information for the staged track in the cache control block for the staged track in response to determining that there is the saved format track information for the staged track.
 15. The system of claim 12, wherein the saving the track format information for the demoted track comprises generating a demoted track cache control block indicating the demoted track and indicating the track format information for the demoted track, and wherein using the saved track format information comprises: generating a cache control block for a track staged into the cache; determining whether there is a demoted cache control block for the track staged into the cache; and including the track format information indicated in the demoted cache control block for the track staged into the cache in the cache control block for the track staged into the cache.
 16. The system of claim 12, wherein the saving the track format information for the demoted track comprises generating a demoted cache control block indicating a track identifier of the demoted track and indicating the track format information for the demoted track, and wherein the operations further comprise: determining an entry in a demoted cache control block directory, having a plurality of entries, based on a track identifier of the demoted track; and indicating the demoted cache control block in the determined entry in the demoted cache control block directory.
 17. The system of claim 16, wherein each of a plurality of entries in the demoted cache control block directory indicate a plurality of demoted cache control blocks whose track identifiers map to the entry in the demoted cache control block directory, wherein the using the saved track format information for the demoted track when the demoted track is staged back into the cache comprises: staging a track from the storage into the cache; generating a cache control block for the staged track; determine an entry of the entries in the demoted cache control block directory based on a track identifier of the staged track; processing indications of demoted cache control blocks in the determined entry for the staged track to determine whether one of the demoted cache control blocks indicated in the determined entry has a track identifier matching the track identifier of the staged track; and including the track format information in the demoted cache control block having the track identifier matching the track identifier of the staged track in the cache control block for the staged track in response to determining the demoted cache control block in the determined entry for the staged track.
 18. A method computer program product for managing tracks in storage cached in a cache, comprising: demoting a track from the cache; indicating the demoted track in a demoted track list; saving track format information for the demoted track, wherein the track format information indicates a layout of data in the track; and using the saved track format information for the demoted track when the demoted track is staged back into the cache.
 19. The method of claim 18, wherein the track format information comprises a track format code defined in a track format table associating track format codes with track format metadata.
 20. The method of claim 18, wherein the track format information comprises a track format code defined in a track format table associating track format codes with track format metadata, wherein the using the saved track format information for the demoted track when the demoted track is staged back into the cache comprises: staging a track from the storage into the cache; generating a cache control block for the staged track; determining whether there is a saved track format information for the staged track that was saved when the staged track was previously demoted; and including the saved track format information for the staged track in the cache control block for the staged track in response to determining that there is the saved format track information for the staged track.
 21. The method of claim 18, wherein the saving the track format information for the demoted track comprises generating a demoted track cache control block indicating the demoted track and indicating the track format information for the demoted track, and wherein using the saved track format information comprises: generating a cache control block for a track staged into the cache; determining whether there is a demoted cache control block for the track staged into the cache; and including the track format information indicated in the demoted cache control block for the track staged into the cache in the cache control block for the track staged into the cache.
 22. The method of claim 18, wherein the saving the track format information for the demoted track comprises generating a demoted cache control block indicating a track identifier of the demoted track and indicating the track format information for the demoted track, further comprising: determining an entry in a demoted cache control block directory, having a plurality of entries, based on a track identifier of the demoted track; and indicating the demoted cache control block in the determined entry in the demoted cache control block directory.
 23. The method of claim 22, wherein each of a plurality of entries in the demoted cache control block directory indicate a plurality of demoted cache control blocks whose track identifiers map to the entry in the demoted cache control block directory, wherein the using the saved track format information for the demoted track when the demoted track is staged back into the cache comprises: staging a track from the storage into the cache; generating a cache control block for the staged track; determine an entry of the entries in the demoted cache control block directory based on a track identifier of the staged track; processing indications of demoted cache control blocks in the determined entry for the staged track to determine whether one of the demoted cache control blocks indicated in the determined entry has a track identifier matching the track identifier of the staged track; and including the track format information in the demoted cache control block having the track identifier matching the track identifier of the staged track in the cache control block for the staged track in response to determining the demoted cache control block in the determined entry for the staged track. 